1. Field of the Invention
The present invention relates to a fluid cylinder capable of moving a main piston by a large fluid force when starting a backward movement.
2. Related Background Art
In general, a fluid cylinder for a die of a die cast machine, an injection machine, etc. works to simply move the die when a piston moves forward (when a piston rod is protruded) and therefore requires no large driving force. When the piston starts moving backward (when starting a retraction of the piston rod), however, there is a necessity for taking off the die from a product in the course of being cooled off. A large driving force is therefore needed.
For this reason, according to the prior art, a pressure intensifying cylinder is interposed between the die-oriented fluid cylinder, a fluid source and a tank, and a fluid discharged out of the fluid source is supplied to the pressure intensifying cylinder. The pressure intensifying piston of the pressure intensifying cylinder is moved toward one side to push out a high-pressure fluid. At the same time, a large force given by this high-pressure fluid pushed out acts to move the piston of the fluid cylinder toward a pull-in side, thus taking off the die from the product. Thereafter, when the pressure intensifying piston moves to a one-side limit, the fluid discharged from the fluid source is supplied to the fluid cylinder via a bypass, and the piston of the fluid cylinder is moved up to a pull-in limit. Next, when the piston of the above-described fluid cylinder is returned to a protrusion limit, the fluid discharged out of the fluid source is supplied to the fluid cylinder to move the piston toward the protrusion side. At the same time, the return fluid flowing out of the fluid cylinder is guided to the pressure intensifying cylinder, thereby moving the pressure intensifying piston toward the other side. At this time, however, a control valve with a throttle that is provided midway of the bypass is opened, and a flow of the return fluid is regulated by the throttle. The return fluid is thereby guided, at first, to the pressure intensifying cylinder, and the pressure intensifying piston is moved up to the other-side limit. Thereafter, the control valve is opened, and the remaining return fluid is returned via the bypass to the tank.
There arise, however, the following problems inherent in the above-explained prior art. The control valve with the throttle is required for determining whether the return fluid should be guided to the pressure intensifying cylinder or allowed to pass through the bypass. The structure is therefore complicated, and, at the same time, the costs increase. Besides, the pressure intensifying cylinder is installed in addition to the fluid cylinder, and, therefore, a large installation space is needed. At the same time, if a pipe for connecting this fluid cylinder to the pressure intensifying cylinder is broken, the high-pressure fluid sprouts out of this broken portion.